Performability is an important parameter in safety-critical real-time systems. This parameter is defined as the joint consideration of two other important parameters, i.e., reliability and performance. This paper proposes a schedulability condition, which guarantees a desired level of performability in various working conditions, for real-time systems. The basic idea underlining this condition is to select a subset of schedulable tasks and manage their slack times to satisfy a desired performability level. The proposed condition is evaluated on a hard real-time system that employs the Rate-Monotonic (RM) scheduling algorithm and uses the re-execution mechanism to improve the reliability. Evaluation results reveal that by employing the condition, the level of performability of the system is always greater than the desired performability. In addition, it yields on average 1% improvement in the system performability in comparison with traditional schedulability conditions, while the actual failure rate is greater than the expected rate. This amount of performability improvement is significant for safety-critical real-time systems.